The invention relates to organic light-emitting diodes, known under the abbreviation OLED, and to a production method of such organic light-emitting diodes.
An organic light-emitting diode is a luminescent component consisting of organic semiconductors. Organic light-emitting diodes consist of one or several thin organic layers which are arranged between electrically conductive electrodes.
An organic light-emitting diode comprises an anode which may, for example, be mounted on a glass plate and which e.g. consists of indium-tin-oxide (ITO) and is transparent within the visible range. Generally a hole transport layer is located above the anode. This hole transport layer may, for example, consist of PEDOT/PSS (poly(3,4-ethylenedioxy thiophene)/polystyrene sulfonate) which is employed for lowering the injection barrier for the holes and as a diffusion barrier for indium. An emitter layer is provided on the hole transport layer, either containing a dye (approx. 5-10%) or entirely consisting of the dye (e.g. aluminum-tris(8-hydroyquinoline), Alq3). An electron transport layer is generally mounted hereon. Finally a cathode with a low electron work function consisting of a metal or an alloy such as e.g. calcium, aluminum, barium, ruthenium, magnesium-silver alloy is for example vapor-deposited under full vacuum. In order to lower the injection barrier for electrons, the cathode will routinely be vapor-deposited as a double layer consisting of a very thin layer of lithium fluoride, cesium fluoride, calcium or barium and a thicker layer of aluminum or silver.
From EP 1083612 A2, cathode layers having a multi-layer structure such as lithium fluoride/calcium/aluminum or barium/silver are known as organic light-emitting diodes. The function of lithium, calcium and barium fluorides, respectively, is to inject electrons into the layer positioned underneath. The thickness of the lithium fluoride layer amounts to a few nanometers. The thickness of the barium and the calcium layers, respectively, may be up to 100 nm. The function of the aluminum or silver layer is to transport the major part of charges from the cathode connector to the light-emitting element that is to the light-emitting layer. The thickness of this layer is in the range of 0.1 to 2 μm. In this arrangement, the layer consisting of lithium fluoride, calcium or barium constitutes the so-called electron injection layer while the aluminum/silver layer will constitute the electrically conductive layer of the cathode. In addition, the aluminum and the silver layers, respectively, protect the still delicate and reactive electron injection layer.
The electrons, i.e. the negative charges, are injected by the cathode while the anode provides holes and defect electrons, respectively, that is the positive charges. Due to an applied electrical field positive and negative charges migrate towards each other and ideally meet within the emitter layer, and that is why this layer is also called recombination layer. By recombination of an electron and a hole in the emitter layer a so-called exciton will be formed. Depending on the mechanism, the exciton either already constitutes the excited state of the dye molecule or the energy of the exciton will be passed to the dye molecule in a transfer process. The dye has several excited states. The excited state may return into ground state while emitting a photon (light particle) during the process. The color of the emitted light depends on the energy gap between excited and ground state and can specifically be changed by means of variation or chemical modification of the dye molecules.
Organic light-emitting diodes are mainly used in displays for computers, TV sets, MP3 players, etc., or as light sources in illumination applications. OLEDs are also increasingly considered as an option for light sources in sensor technology applications.
If organic light-emitting diodes are to be applied in full color displays it is required that at least three basic colors can be generated independently controlled of each other. The display then comprises a multitude of single pixels and organic light-emitting diodes, respectively, capable of emitting light of different colors, wherein the generation of light by each pixel can be electronically controlled.
In the case that an organic light-emitting diode is to be used as a light source for illumination an electronic control of single pixels is not required. Although a light source may also comprise a multitude of pixels or organic light-emitting diodes, respectively, which can generate different light colors this distribution merely is to allow generating the desired light color, such as, for example, white light, by having the pixels emit the three basic colors red, green and blue in a suitably distributed manner.
If organic light-emitting diodes are to be applied in sensor technology it depends on the respective application whether separately controllable pixels emitting different colors are required or not. Due to the spectrally wide emission—in comparison to other light sources used in sensor technology (e.g. LEDs)—of most organic light-emitting diodes known from the state of the art, the use of pixels emitting different colors has generally not been feasible in a suitable manner in prior art.
It is known from the state of the art that the single pixels of a display or a light source based on organic light-emitting diodes can be produced under full vacuum by thermal evaporation of small molecules emitting different colors. In said at least three step process red-, green- and blue-emitting small molecules are for example successively applied to the substrate by means of shadow masks. In this process basically only small molecules and no polymers will be deposited since polymers are too large for thermal evaporation.
A typical small molecule which is deposited by thermal evaporation is the metal complex Alq3. Alq3 is able to emit green light. By adding dyes the emission color of Alq3 can be changed. Additionally, other metal complexes or other small molecules may also be used in order to generate other colors which cannot be achieved with Alq3 or modified Alq3.
Thermal evaporation is preferred since there is no need to use solvents although employing such a process is of high cost. If a solvent is used the risk of contaminating the material will exist compromising efficiency of an organic light-emitting diode and duration of life thereof.
A shadow mask which is used for the disclosed structure resembles a grid. The holes of the shadow mask are located in places which the respective molecule is to reach on a substrate located underneath. After a first deposition of first molecules through the holes of the shadow mask the shadow mask is moved in a suitable manner or replaced by a second shadow mask and second molecules which emit another light color are deposited onto the substrate positioned underneath through the holes of the shadow mask. In the same manner, third molecules will be deposited in a third step.
Proper alignment of the shadow masks in relation to each other and in relation to the substrate has proven to be difficult since alignment precisions in the order of 1 to 10 μm are required over large surfaces [Stephen R. Forrest, Nature 428, p. 911-918 (2004)]. An additional problem is that during deposition the molecules are not only deposited on the substrate but also, disadvantageously, on the shadow mask. This deposition causes a change of the holes in the shadow mask and thus will lead to a change of depositions on the substrate. Furthermore, the deposited substances may detach from the shadow mask resulting in dust formation. Dust formation will disadvantageously affect the manufacture of organic light-emitting diodes.
From the document “Solution-Processed Full-Color Polymer Organic Light-Emitting Diode Displays Fabricated by Direct Photolithography” by Malte C. Gather, Anne Kohnen et al. in Advanced Functional Materials, 2007, 17, 191-200, an alternative photolithographic method is known for the manufacture of organic various color light-emitting diodes in in the form of pixels of a display, as described herein. The emitting materials—in this case electroluminescent polymers—are deposited from a solution, so that the costly step of thermal evaporation under full vacuum can be omitted. However, here again, three consecutive alignment steps are required to produce pixels emitting red, green and blue.
In order to avoid the complicated alignment process it has been suggested to use a blend of material or a multiple layer design which emits white light, and to use it for all subpixels instead of structured deposition of small molecules or polymers emitting various colors. Suitable blend of material/multiple layer designs may consist of polymers or small molecules such as e.g. metal complexes. In the latter case metal complexes are frequently preferred since they currently allow achieving a higher light intensity. Generally, such a blend of material and multi-layer design, respectively, comprises components which emit the three basic colors, thus ultimately emitting white light.
Color appearance is achieved by post-lamination of a matrix of color filters. Such a matrix or color filter array, respectively, is made of pigmented photoresists. In the production process a first photoresist layer is initially applied which, for example, is employed as a color filter for blue light. This photoresist layer will be exposed to light in the respective areas and will thus be rendered insoluble. The remaining unexposed areas are rinsed off. Subsequently, an additional photoresist is applied which forms another color filter. Thus, in at least three steps the desired matrix of color filters is obtained. Such state of the art can, for example, be found in the document “I Underwood et al., SID 04 Digest, p. 293-295 (2004)” and “B. J. Green, Displays 10(3), p. 181-184 (1989)”.
However the alignment problem will only be delayed in the process chain as the color filters and the color filter array, respectively, must exactly be adjusted in relation to the subpixel structure. Additionally color saturation and performance which are achieved using this method will not be sufficient for many applications.
From the document US 2007/0286944 A1 an organic light-emitting diode is known which comprises a lossy optical resonator. A layer system consisting of a hole injection layer, a hole transport layer, an emitting layer, and an electron transport layer is located between a completely reflective cathode layer and a partially reflective anode layer. The light color which is generated by this organic light-emitting diode is adjusted by means of distance between the two reflective electrode layers. In order to generate different light colors the thickness of the hole injection layer is varied. The various layers are applied by vapor deposition. In order to adjust the thickness of the hole injection layer, small molecules are vapor-deposited in different thicknesses. In order to produce three different defined layer thicknesses it is necessary to use and suitably align shadow masks in the vapor coating process. Therefore, this state of the art has the disadvantages already mentioned above, i.e. there will be the necessity to perform several complicated alignments of the mask.
The document U.S. Pat. No. 6,091,197 also discloses an organic light-emitting diode having a lossy optical resonator wherein a layer capable of generating white light is arranged. The distance between the two reflective layers of the optical resonator defines the color of the light emanating from the organic light-emitting diode. This distance can be varied by means of an electromechanical control in order to change the light color of the organic light-emitting diode. Although the state of the art known from the document U.S. Pat. No. 6,091,197 is suitable to produce a light source which can be adjusted to a desired light color which can be changed at any time, the invention known therefrom is not suitable for other applications. For example the device is not suited for the use as a display because an electrical field has to be permanently applied in order to maintain a desired emission color. The manufacture and alignment of the flexible membrane is quite elaborate, such that neither generation of a cost efficient white light source, a light source for sensor applications emitting light in various colors, nor a spectrometer, are feasible by the use of this device.
From the document DE 100 37 391 A1 cross-linkable organic materials are known which are to be used in organic light-emitting diodes.